I'm speaking here about GW LoW, not FW.
Several armies, almost all main ones, have their LoW unit, some have severals. I'm really surpised that such a popular army like tyranid not having one. Even worse, the FW one is pysically not usable, not convenient (too big, too fragile, no base etc...).
I think that Tyranids need one proper Lord of War. A tall bug with at least 2 builds and a bunch of weapons. In term of sales, I'm pretty sure that it'd be a bargain for GW. They'd sell tons of them and they'd make even more tyranids sales. A kit like a IK kit, a mini the size of a knight or taller if standing (like a bigger mawloc) and larger if rampant (like a bigger exocrine/tyranofex).
So, why do you think that it never happened before? Why no LoW?
As a side note, I'd be glad to see also some kind of super brainy bug (like in Starship troopers) with special abilities. (but not named char, please)